Chemical generation or mixing systems are used in a variety of industrial applications to blend two or more components or constituents to a desired concentration. In semiconductor fabrication facilities, for example, concentrated chemicals (which are usually provided by commercial chemical suppliers in solution with water) are commonly mixed or diluted with DI (deionized or ultra pure) water before being sprayed on or otherwise applied to semiconductor wafers. Table 1 below lists a number of chemicals used in semiconductor fabrication facilities, and the concentration (in weight %) in which these chemicals are typically provided by suppliers.
TABLE 1 ______________________________________ Percentage Concentrate Chemical Symbol in Water ______________________________________ Hydrofluoric Acid HF 49% Acetic Acid HAC 99.7% Nitric Acid HNO.sub.3 71% Phosphoric Acid H.sub.3 PO.sub.4 80% Potassium Hydroxide KOH 30% Tetramethyl Ammonium TMAH 25% Hydroxide Hydrochloric Acid HCl 37% HF and Ammonium BOEs -- Fluoride Mixtures Ammonium Hydroxide NH.sub.4 OH 28-30% Sulfuric Acid H.sub.2 SO.sub.4 93-98% ______________________________________
When used in semiconductor fabrication facilities, the concentrated chemicals described above are commonly diluted with DI water (i.e., a diluent) to desired concentrations or assays. Concentrations in these applications are typically described in terms of weight % (weight percent) of concentrated or pure chemical in water. Hydrofluoric Acid (HF), for example, is often diluted with ultra pure water to concentrations ranging from about 0.5%-5% HF by weight when used for etching and cleaning processes. Tetramethyl Ammonium Hydroxide (TMAH) is often diluted to about 2.38 weight % for use as a positive photoresist developer. Non-aqueous blended chemicals, and blended chemicals with three or more components, can also be generated.
Chemical mixing systems blend the chemicals to a desired concentration which is sometimes known as the nominal or qualification concentration. A high degree of accuracy is also required. The range or window of acceptable concentrations surrounding the qualification concentration is known as the qualification range, and can be defined as a weight % error with respect to the qualification concentration, or by upper and lower qualification range concentrations. Chemical blending systems of the type described above are commercially available from a number of sources including FSI International of Chaska, Minn. and Applied Chemical Solutions of Hollister, Calif.. They are also disclosed generally in the Geatz U.S. Pat. No. 5,148,945 and the Ferri, Jr. et al. U.S. Pat. No. 5,330,072.
There remains, however, a continuing need for improved chemical blending systems. In particular, there is a need for chemical blending systems capable of quickly blending batches of chemical to a very high degree of accuracy. To be commercially viable, the chemical blending systems must also be highly reliable.